Many breeding studies have been conducted to improve the fatty acid profile of Brassica varieties. Pleines and Freidt, Fat Sci. Technol., 90(5), 167-171 (1988) describe plant lines with reduced C18:3 levels (2.5-5.8%) combined with high oleic content (73-79%). Rakow and McGregor, J. Amer. Oil Chem. Soc., 50, 400-403 (Oct. 1973) discuss problems associated with selecting mutants for linoleic and linolenic acids. In Can. J. Plant Sci., 68, 509-511 (Apr. 1988) Stellar summer rape producing seed oil with 3% linolenic acid and 28% linoleic acid is disclosed. Roy and Tarr, Z. Pflanzenzuchtg, 95 (3), 201-209 (1985) teaches transfer of genes through an interspecific cross from Brassica juncea into Brassica napus resulting in a reconstituted line combining high linoleic with low linolenic acid content. Roy and Tarr, Plant Breeding, 98, 89-96 (1987) discuss prospects for development of B. napus L. having improved linolenic and linolenic acid content. European Patent application 323,753 published Jul. 12, 1989 discloses seeds and oils having greater than 79% oleic acid combined with less than 3.5% linolenic acid. Canvin, Can. J. Botany, 43, 63-69 (1965) discusses the effect of temperature on the fatty acid composition of oils from several seed crops including rapeseed.
Mutations typically are induced with extremely high doses of radiation and/or chemical mutagens (Gaul, H. Radiation Botany (1964) 4:155-232). High dose levels which exceed LD50, and typically reach LD90, led to maximum achievable mutation rates. In mutation breeding of Brassica varieties high levels of chemical mutagens alone or combined with radiation have induced a limited number of fatty acid mutations (Rakow, G. Z. Pflanzenzuchtg (1973) 69:62-82). The low α-linolenic acid mutation derived from the Rakow mutation breeding program did not have direct commercial application because of low seed yield. The first commercial cultivar using the low α-linolenic acid mutation derived in 1973 was released in 1988 as the variety Stellar (Scarth, R. et al., Can. J. Plant Sci. (1988) 68:509-511). Stellar was 20% lower yielding than commercial cultivars at the time of its release.
Alterations in fatty acid composition of vegetable oils is desirable for meeting specific food and industrial uses. For example, Brassica varieties with increased monounsaturate levels (oleic acid) in the seed oil, and products derived from such oil, would improve lipid nutrition. Canola lines which are low in polyunsaturated fatty acids and high in oleic acid tend to have higher oxidative stability, which is a useful trait for the retail food industry.
Delta-12 fatty acid desaturase (also known as oleic desaturase) is involved in the enzymatic conversion of oleic acid to linoleic acid. Delta-15 fatty acid desaturase (also known as linoleic acid desaturase) is involved in the enzymatic conversion of linoleic acid to α-linolenic acid. A microsomal delta-12 desaturase has been cloned and characterized using T-DNA tagging. Okuley, et al., Plant Cell 6:147-158 (1994). The nucleotide sequences of higher plant genes encoding microsomal delta-12 fatty acid desaturase are described in Lightner et al., WO94/11516. Sequences of higher plant genes encoding microsomal and plastid delta-15 fatty acid desaturases are disclosed in Yadav, N., et al., Plant Physiol., 103:467-476 (1993), WO 93/11245 and Arondel, V. et al., Science, 258:1353-1355 (1992). However, there are no teachings that disclose mutations in delta-12 or delta-15 fatty acid desaturase coding sequences from plants. There is a need in the art for more efficient methods to develop plant lines that contain delta-12 or delta-15 fatty acid desaturase gene sequence mutations effective for altering the fatty acid composition of seeds.